3.1. Near-infrared observations
First, we compare the morphology of BBW 192E at NIR and optical wavelengths. In Fig. 1a, we present an K´ band image overlayed with contours of the optical emission, drawn from the DSS. In addition, we numbered each stellar-like object for which we performed photometry to ease identification in Table 2 (photometric results). At optical wavelengths, BBW 192E is extended essentially in southeast-northwest direction with the IRAS source located at the bright southeastern edge. Figs. 1b and 1c are subfields (45" x 45") of the H and K´ band images which include the close environment of BBW 192E. The NIR images of BBW 192E show a pronounced bipolar nebula now with a northeast-southwest orientation. The centre of the bipolar infrared nebula in BBW 192E is located about 12" southeast of the peak of the optical emission.
Fig. 1b shows an H band image of the bipolar infrared nebula. The lobes of this nebula are separated by a curved less bright lane. Figs. 1c and e display the K´ and Ks images. Whereas the north-eastern lobe of the infrared nebula appears brighter than the south-western lobe in the K´ and Ks images, it seems to be fainter in the H band image. Indeed, our photometry reveals that the flux ratio between the north-eastern and south-western lobe changes from 0.8 (H) to 1.5 (K´). After the subtraction of stellar-like contributions from the K´ image, especially at the position of object 25, the morphology in the K´ image is similar to that in the H image (see Figs. 1d and 1b). The K´ and Ks band images (Fig. 1c and e) clearly show that object 20, which is located close to the less bright band in the H band image, is not directly related to the bipolar infrared nebula.
Fig. 1e shows BBW 192E at the highest angular resolution and, in addition, reveals the distribution of the linear polarization of the NIR emission. The core of the intensity profile of the bright source 25 can be well matched by the PSF derived from the surrounding stars. The global polarization pattern, except the most eastern part, of the nebula is centro-symmetric. This feature and the high polarization degree indicate the dominance of single scattering in this area. The parallel polarization pattern in the eastern part of the nebula indicates a dust component influencing the polarization structure of the bipolar lobes. The polarization structure at source 25 together with its PSF leads us to the conclusion that the central star of the bipolar nebula can be seen directly. The orientation of the polarization vectors for regions dominated by single scattering (i.e with large polarization degrees ) was used to derive the location of the illuminating source. This was done by maximizing the sum over all scalar products between the polarization and radius vectors as a function of the source location. The resulting position, indicated by the cross in Fig. 1e, coincides within the formal errors of this method with that of object 25. Thus, we conclude that this source is the major illuminator which produces both the near-infrared and optical scattering nebulosities.
The surface brightness of the total bipolar nebula as well as of its individual lobes was estimated from H and K´ images which were cleaned from the stellar contributions. The average surface brightness of the total bipolar nebula is 18.0 and 16.9 mag/arcsec2 for the H and K´ band emission, respectively. The average surface brightness of the south-western lobe (area: 660 arcsec2) amounts to 17.1 and 16.5 mag/arcsec2, that of the north-eastern lobe (area: 320 arcsec2) to 18.2 and 17.4 mag/arcsec2 for the H and K´ band, respectively.
The single-channel NIR photometry (15" aperture) of BBW 192E by Liseau et al. (1992) led to the detection of only two sources (IRS 26/1 & 26/2), about 5" and 26" offset from the IRAS position. For comparison, we present both single-channel photometry as well as the photometry derived from our NIR images using a synthetic aperture of 15". These measurements close to IRS 26/1 included the objects 20 and 25 as well as parts of the bipolar infrared nebula. Table 1 compiles our single-channel measurements of this region (see Fig. 1c).
Table 1. 15" aperture single-channel photometry of BBW 192E
Our JLM-magnitudes are in good agreement with those of Liseau et al. (1992). However, our H and K magnitudes differ from those of Liseau et al. (1992) by 0.3 mag. The photometry at the position of IRS 26/1 and IRS 26/2 in our H and K´ band images resulted in 11.9/10.4 mag (26/1) and 11.3/10.2 mag (26/2), respectively. The synthetic-aperture photometry on the H and K´ band images provides higher flux densities for the two sources than estimated from the single-channel measurements. Therefore, we conclude that some of the single-channel measurements were actually not performed at the position of the maximum K´ band emission. Since the target region is more extended in the H and K´ bands than in the L and M bands, such position uncertainties will cause higher deviations of the flux densities in the H and K´ bands.
Our images show that the single-channel photometry included several objects (see Figs. 1a, c). At least 7 stellar-like objects are located within a circle of 20" radius around the nominal position of the IRAS source. Only one faint source ( 20, see Fig. 1a) lies within the IRAS error ellipse. Another faint and optically not visible source ( 18) is located at the northern border of the error ellipse.
The photometric data of 32 individual objects obtained from the images (HK´) are summarized in Table 2. Most of the objects have a stellar-like PSF. Source 20 could not be resolved in the H band since it is embedded in a band of extended emission. Object 5 is identical with ESO-H-259 for which Pettersson & Reipurth (1993) found faint H emission.
Table 2. H, K´ band photometry of individual objects
3.2. Mid-infrared imaging
In Figs. 2a, b, c and e we present N and Q band images of BBW 192E, respectively. It is remarkable that we detected extended emission with evidence for a bipolar morphology in both bands. The morphology of the N band image is quite similar to that of the K´ image. We did not detect stellar-like objects in the MIR images outside the bipolar infrared nebula. Therefore, we do not have objects with known astrometric positions within the MIR images. The coordinate system of the N band image was established by assuming that the stellar-like component in the north-eastern lobe of the K´ image (object 25) corresponds to the stellar-like object in the N band. There is an offset between the peaks of the N and Q band emission of . The absence of stellar-like sources outside the nebula suggests that object 25 is the most luminous object in the investigated region.
For the N and Q band images, magnitudes of 2.10.1 and -1.70.4 mag (6.2 Jy and 48.7 Jy) were estimated within a 15" aperture around the position of source 25, respectively. In the case of a 22" aperture the magnitudes amount to 1.60.1 and -2.00.4 mag (9.3 Jy and 64.2 Jy) for the N and Q band, respectively. The IRAS point source flux densities of IRAS 08513-4201 are 12, 135, 455, and 618 Jy at 12, 25, 60, and 100 µm. The flux densities we measured are well below the IRAS flux densities indicating the presence of extended emission.
To separate the flux density contributions of source 25 and the bipolar nebula in the N band, object 25 was removed from the image by subtracting a properly scaled image of a standard star. The brightness of source 25 was estimated to be 3.50.2 mag. In addition, the magnitudes of the north-eastern and south-western bipolar infrared lobes (N band) are 3.30.1 and 3.00.1 mag, respectively. The average surface brightness which was measured above the 5 contour of the north-eastern (area: 88 ") and south-western (area: 105 ") bipolar lobes amounts to 8.6 and 8.4 mag/ ", respectively.
3.3. 1.3 mm continuum observations
Fig. 2d shows the 1.3 mm continuum emission as contours overlayed on the H band image. We detected millimetre emission in an area of 151" x 55" with the peak position approximately at the position of IRAS 08513-4201. The sizes represent the main axes of an elliptical configuration which includes the same area as the 3 contour of the millimetre emission. For their determination we deconvolved the main axis of the elliptical configuration with the beam size. This guarantees the estimation of reliable physical parameters from the map. The total flux density and the peak flux density were estimated within the 3 contour to be 4.1 and 0.6 Jy, respectively (root mean square noise: 66 mJy).
The millimetre emission has a cometary shape (see dashed line in Fig. 2d). In addition, two faint extensions are visible in south-east direction. The bipolar infrared nebula is located at the peak of the millimetre emission.
The total flux density estimated from the 1.3 mm map was used to derive the hydrogen column () and number () densities (both averaged over the source), and the gas mass () of BBW 192E using the formula given in Henning et al. (1998). The 1.3 mm dust continuum emission was assumed to be optically thin, which may not be valid, however, for the inner very dense disk-like structure. The 4.85 GHz continuum emission of BBW 192E due to free-free transitions is below the detection limit of 25 mJy of the Parkes-MIT-NRAO survey (Gregory et al. 1994). Thus we conclude that the observed 1.3 mm continuum emission is almost entirely thermal radiation from cold dust. For the derivation of the physical parameters, we used a mass absorption coefficient of 0.5 cm2g-1 (Ossenkopf & Henning 1994), and an hydrogen-to-dust mass ratio of (Draine & Lee 1984). The dust temperature was estimated to be 35 K by a black-body fit (modified with the wavelength dependence of the opacity) to the spectral energy distribution (see Fig. 4). Accounting for He and metals, the total gas-to-dust mass ratio is (/) 150/() where we assumed solar metallicity ( = 1). For the calculation of the number density, we assumed a spherical symmetric morphology for which a cut through the center has the same area as the millimetre source in the plane of sky.
We derived a total gas mass of about 180 around BBW 192E. This value is quite high compared to the average mass (8060 ) of core/envelope structures associated with Herbig Ae/Be stars (HAEBEs, Henning et al. 1998). On the other hand, the mass of BBW 192E is low compared to that of the HAEBE star Cod-42o11721 with the highest total mass (1100 ) as derived by Henning et al. (1998). The source-averaged hydrogen number density of BBW 192E amounts to 2.6106cm-3 and lies at the upper end of the density range of the envelopes around HAEBE and FU Orionis stars (105 to 108 cm-3 for the cores and 104 to 105 cm-3 for envelopes, see Henning et al. 1998). The average hydrogen column density was estimated to be 4.51022cm-2.
© European Southern Observatory (ESO) 2000
Online publication: December 8, 1999